Some of the challenges only require one participant total to complete and some of them require multiple participants, if that's what you mean. So on ones that say "a participant [does something]," then that challenge will be marked complete if one participant completes a work that fills the criteria.

On ones that say "complete [multiple works/treats]" theoretically one participant could complete that challenge on their own, but I was thinking those would work across the exchange as a whole, which would more likely mean multiple participants completing one work each that fits the criteria.

I hope that helps. If not, I'm happy to clarify further.